Description:
------------
The json_encode() function completely ignores, and thus does not encode, 
PHP array entries with an array key that is an empty string: array
(""=>"value");

Empty string array keys and object properties are totally valid PHP and 
JSON respectively - they should encode/decode as such.

This, along with the corresponding json_decode() bug of a similar nature  
(41504), are show stoppers for me as I need JSON encode/decode to be 
rock solid and consistent.

Thanks for looking into this :-)

Reproduce code:
---------------
1) var_dump(json_encode(array(""=>"value")));
2) var_dump(json_encode(array(""=>"value1", "key2"=>"value2")));

Expected result:
----------------
1) string(15) "{"":"value"}"
2) string(17) "{"":"value1", "key2":"value2"}"

Actual result:
--------------
1) string(2) "{}"
2) string(17) "{"key2":"value2"}"

In both cases the array entry with the empty string key is missing.

This might be addressed by Bug #41034, but may very well be a different 
bug.